Magnetic resonance imaging (MRI) can be used in various medical diagnosis procedures. In general, MRI is based on measuring radio frequency (RF) emissions of water protons in a magnetic field following exposure to an external RF pulse. Parameters used in constructing an image during MRI include the time that it takes for the water protons to relax to their equilibrium state following an RF pulse. The relaxation of water protons following an external RF pulse has two component parameters, designated T1 and T2. Paramagnetic contrast agents affect the T1 component in the relaxivity of water protons, effectively enhancing the contrast during the MRI procedure. On the other hand, ferromagnetic contrast agents affect the T2 component in the relaxivity of water protons. For many applications, agents affecting T1 are preferred because the enhancement provides a brighter image relative to the background.
Elements that have magnetic or paramagnetic properties and cause nearby protons to have a short relaxation time, such as various transition metals, can be used to enhance contrast during MRI procedures. Gadolinium (Gd), with seven unpaired electrons, is particularly effective at enhancing the relaxivity of water during MRI.